Python interpreter and libraries/headers detection.

- The user can now pass its own interpreter.
- Development libraries and headers can be requested.
- Travis-CI switched to new container-based workers.

test/python_interpreter_custom/cmake/autocmake.cfg

@@ -0,0 +1,14 @@
+[project]
+name: example
+
+[cxx]
+source: ../../../modules/cxx.cmake
+
+[python_interpreter]
+source: ../../../modules/python_interpreter.cmake
+
+[default_build_paths]
+source: ../../../modules/default_build_paths.cmake
+
+[src]
+source: ../../../modules/src.cmake